Sensitivity of energy balance and implications

This work has addressed the formation of penitentes through an experimental and modelling campaign in the

Variation of EB components with height

Variation of EB components with height

Figure 3.8 Variation of the energy balance components with height for mean recorded values at the lower AWS (3335 m a.s.l.) applying a standard lapse rate (-0.0065 Km-1). Short-wave global radiation is modified primarily by albedo, which in this case was simplified to a constant value

Andes. Penitentes are a unique and complex form that result from a relatively narrow band of meteorological conditions. Thus, any change in climate is likely to have implications for the formation of penitentes, and thus in turn the mass balance of glaciers as their buffering effect on snow melt, as discussed in the previous section, is reduced.

A simple study of the sensitivity of the energy balance to different parameters and its variation with height was carried out. This is represented in Figure 3.8. We can observe a minimum of the net energy balance at about 4600 m a.s.l., which corresponds well with the maximum extension of penitentes. The shape of the curve is relatively sensitive to initial temperature and relative humidity and very sensitive to wind speed. Wind speed was fairly constant and moderate to light at the upper AWS, with values that are very similar to recorded values in a previous campaign in the Argentinian Andes at similar latitude and height. Increased wind speed decreases dramatically the net energy balance and would prevent the formation of penitentes, as the turbulent fluxes are proportional to roughness. This was observed on the upper section of the Juncal glacier, where wind is higher on unsheltered slopes and snow is less metamorphosed and relatively smooth. However, penitentes were found not far below the summit of Nevado Juncal (6100 m) in a very sheltered location. A similar situation was observed on Cerro Aconcagua, where the snow ablated into penitentes up to an altitude of 5800 m except on the very wind-exposed eastern section (Polish glacier), where flatter snow or bare ice existed. Running the model with a dry adiabatic lapse rate, as may be expected under a katabatic wind regime, results in a shift of the minimum in energy balance to a lower altitude. The same situation could be expected earlier in the season, when air temperature is lower. Observations on Juncal glacier confirmed this, the lower line of penitentes formation migrated upwards from about 3700 m in early December to about 4000 m later in the season. Small penitentes formed at lower elevations gradually became wet, rounded and disappeared over a period of a few weeks.

Through this set of experimental and modelled data, we can make a number of observations about the impact of changes in meteorological variables as follows.

• Increased humidity will hinder the formation of penitentes, both by decreasing the latent heat flux and increasing the net long-wave radiation.

• Increased temperature will shift upwards the lower limit of penitentes formation.

• Stronger circulation with increased wind speeds will decrease or suppress penitentes formation.

This initial sensitivity study does not allow us to comment on the likely implications of climate change on penitentes and subsequent influences on glacier energy and mass balance. Rather, it demonstrates that penitentes are sensitive to changes in meteorological parameters and that the model developed allows us to form some initial hypotheses about the likely impacts. Further work, including the extensive collection of field data in stable and unstable atmospheric conditions, should shed further light on these processes.

As we have seen, the formation of penitentes occurs within a narrow band of climatic conditions, and their presence provides information on seasonal trends, a point already stressed by Kotlyakov and Lebedeva (1974), thus, one direct application of the relationship between snow surface morphology and climate is the potential use of remote sensing for assessing seasonal climatic conditions. The increased roughness of penitentes is potentially detectable using SAR polarimetry, while changes in albedo may identify the differences between flat snow and penitentes areas through the use of optical remote sensing. To exploit the full potential of this relationship, a better knowledge of the initial stages in the formation of micropenitentes is needed, and that was beyond the scope of this work. Far more detailed micrometeorological measurements in the field or the replication of the process in a cold laboratory under controlled circumstances would be necessary to gain full insight into this process of snow ablation.

3.7 CONCLUSIONS

The climatic characteristics of the Dry Central Andes - low humidity and high solar radiation inputs in stable summers coupled with high evaporation rates and strong radiative cooling - result in a unique snow ablation morphology: penitentes. Modelling and field data suggest that any changes in the meteorological conditions during the initial stages of growth of penitentes early in the ablation season, for example, increased humidity, long-wave radiation or stronger winds, may suppress or hinder their formation. Since the growth of penitentes is self supported in part by a positive feedback mechanism, the consequences of a small change in meteorological conditions may result in a disproportionate change in overall ablation. Modelling work suggests that penitentes enhance conservation of snow cover, and the consequences of their loss might be increased ablation over the whole season, decreased glacier mass balance and faster depletion of water resources. Given the critical nature of snow and ice melt in relation to water resources for human consumption and agricultural resources in the Central Andes, any potential change is worthy of further research.

3.8 ACKNOWLEDGEMENTS

Field work was possible thanks to the support of Professor David Sugden and a research grant from the Carnegie Trust while J. G. Corripio was enjoying a Carnegie Scholarship at the University of Edinburgh. We are grateful for the help given by the Laboratorio de Glaciología, University of Chile, and especially to Andres Rivera and Jorge Quinteros. Fieldwork was easier and more enjoyable thanks to dedicated field assistants Cameron Thomson and Carlitos Gomez. We would like to thank the Chilean Dirección General de Aguas and acknowledge the support of the ETH, Zurich, especially the Arolla group: Uli Strasser, Francesca Pellicciotti, Paolo Burlando, Martin Funk and Ben Brock. The final version of this paper was improved thanks to the helpful comments and suggestions of the anonymous reviewers.

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